The present invention relates to a radio communication system applied to, for example, an indoor radio LAN system so as to transmit and receive the data between a base station and a plurality of terminal stations in the frequency hopping spread spectrum system.
When the radio communication is made between the base station and the terminal stations in an indoor space, for example, it is strongly influenced by the fading caused by the multipass (a multiple reflection wave) and, therefore, transmitting and receiving the data while adjusting the directivity in a direction in which a maximum receiving wave comes is effective. To implement this, there is a system capable of surely performing the radio communication between the base station and the terminal stations by using adaptive array antennas and optimally controlling the directivity while varying weight values of the outputs of the respective antennas.
For example, the system of Jpn. Pat. Appln. KOKAI Publication No. 9-219615 comprises adaptive array antennas 1 composed of a plurality of antenna elements, weighting units 2 for multiplying transmitting and receiving signals of the respective antenna elements by coefficients of weighting that have been set and thereby weighting the amplitudes and phases, a dividing/combining unit 3 for distributing the transmitting signals to the respective antenna elements via the weighting units 2 and synchronizing the receiving signals from the antenna elements, a transmission/reception system 4, an interface 5 and an external operation unit 6, as shown in FIG. 11. The external operation unit 6 calculates the adaptive control of the directivity for every terminal of a communication partner at a non-real time, assigns in advance the time slots to the respective terminals at the time of transmission and reception, and switches the weight values for the respective terminals at time division.
Each weighting unit 2 varies the phase and the amplitude in a transmitting signal supplied from the dividing/synthesis unit 3 by predetermined amounts or multiply the signal by a complex weight. Thus, the radio waves radiated from the antenna elements form a desired synthetic transmission directional pattern.
On the other hand, the phase and the amplitude of each of the signals received by the plural antenna elements are controlled by the weighting units 2 or the signal is multiplied by the complex weight. Then, the signals are combined by the dividing/synthesis unit 3. The desired receiving directional pattern can be formed in this manner.
The system of this publication calculates in advance the directional adaptive control of the array antennas by the external operation unit 6 and controls the directivity by using the weight values that have already been calculated at the time of the communication with each terminal. Therefore, the system can be structurally simplified and miniaturized at lower costs as compared with a system that calculates the weight value at a real time (sequentially) during the communication.
Incidentally, the frequency hopping spread spectrum modulation that hops the frequency within a certain frequency band is known as one of the modulation systems. If the radio communication is made in such a frequency hopping spread spectrum modulation in the system of the above publication, the directivity of the array antennas is controlled by using the weight values that are calculated on the basis of a certain carrier frequency, for example, a center frequency in the carrier frequency band.
However, the frequency band used in the frequency hopping is wide. Therefore, when the communication is made with the frequencies at the edges of the frequency band.
The object of the present invention is to provide a radio communication system for performing radio communication between a base station and terminal stations in the frequency hopping spread spectrum, which allows the radio communication to be certainly made at any time between the base station and each of the terminal stations even when the hopping is executed, and which has a simple structure.
Another object of the present invention is to provide a radio communication system which allow the control of the optimum directivity to sufficiently follow the change of the radio propagation environment and can thereby perform preferable radio communication.
To achieve these objects, according to one aspect of the present invention, there is provided a radio communication system including a base station and a plurality of terminal stations each communicate with the base station by radio in a frequency hopping spread spectrum. The base station includes a radio communication apparatus comprising a receiving section for demodulating a signal obtained by multiplying an amplitude and a phase of each of receiving signals from a plurality of antenna elements by weight values and synthesizing the signals, and a transmitting section for distributing the modulated signal into a plurality of signals, multiplying an amplitude and a phase of each of the divided signals by weight values, and radiating the signals from the respective antenna elements. The base station comprises weight value storing means for storing the weight values of the receiving section and the transmitting section in response to each of the terminal stations and each of hopping frequencies, and directivity controlling means, every time a carrier frequency is hopped at the time of performing radio communication with each of the terminal stations, for reading the weight values of the transmitting section and the receiving section responding to the frequency and each of the terminal stations from the weight value storing means, and controlling the directivity of the transmitting section and the receiving section.
The system of the present invention further comprises error rate detecting means for detecting variation in a data error rate in the signal received by the receiving section, for each of the terminal stations, and weight value rewriting means, when the error rate detecting means detects increase in the error rate of a terminal station, for calculating again and rewriting the weight values of the receiving section and the transmitting section corresponding to the terminal station in the weight value storing means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.